Active studies have recently been made on an ink jet printing technology using an energy beam such as ultraviolet radiation (refer to JP-A 2004-526820). This printing technology is to apply a liquid ink to the surface of a material to be printed such as paper or plastic by an ink jet system and apply, for example, ultraviolet radiation to the ink to crosslink or cure it. Since this printing technology has advantages that the ink is dried quickly and does not contain a solvent and that a material to be printed which does not absorb an ink can be printed unlike a conventional printing technology in which an ink is crosslinked or cured by heat, it is attracting attention.
For ink jet printing, the ink desirably has a viscosity of 30 mPa·s or less at normal temperature to ensure continuous ejection stability from a nozzle head and prevent the clogging of the nozzle head. However, since a high-viscosity component such as a polymer or crosslinking agent must be used to increase the strength of a film after crosslinking or curing, the reduction of viscosity and the strength of the film have a trade-off relationship. Although an acrylic ester has been used as a reactive diluent to reduce the viscosity of a printing ink, a low molecular weight acrylic ester having a molecular weight of 300 or less generally has high skin irritancy and accordingly, its amount is limited. Therefore, there has been limitation to the reduction of viscosity with the acrylic ester. Then, there is proposed use of a methacrylic ester having low skin irritancy and a low molecular weight. However, as the methacrylic ester generally has a lower curing rate than the acrylic ester, it is disadvantageous in terms of productivity, and the strength of the cured film is not satisfactory.